The present invention relates to a method for etching a bar code on a metal surface, such as the surface of a tool. The invention also relates to a sheet having a plurality of adhesive etching masks that can be cut from the sheet and used to practice the method, with the different masks of the sheet providing bar codes which encode different information such as consecutive serial numbers to be etched onto tools.
A bar code is an array of bars which are marked parallel to one another on a surface and which have various thicknesses to encode information such as characters and/or numbers. The encoded information can be optically read, or converted into electrical signals for use by a computer or other circuitry, using a hand-held wand or a counter-installed scanner. The Universal Product Code is a bar code that is familiar to most people in the United States because it has been adopted as a standard by the food industry, and is frequently printed on labels for cans or boxes of prepackaged food. In one version of this code, ten digits can be encoded to identify such features as the manufacturer, brand, and size. Specifications for the Universal Product Code establish the number and thickness of the bars needed to encode these ten digits, together with other information such as stop and start codes and a check digit. Despite the ubiquity of the Universal Product Code in the grocery field, however, specifications have been published to provide formats for other bar codes. For example Code 39, which has been adopted by the United States Department of Defense and which is being increasingly used by industry, employees 18 bars to encode 6 digits and ancillary information.
The digits permitted by a particular bar code format might be encoded using pen and ink by a draftsman who pays close attention to the code specifications. However it is far more frequent to use a bar code printer. Bar code printers are available which employ keyboards for manual input of the data to be encoded. One such bar code printer is the C. Itoh model CI-800, which is available from CIE Terminals, Inc., 2505 McCabe Way, Irvine, Calif. 92714. Furthermore commercially-available programs for printing bar codes can be run on personal computer systems having dot-matrix printers.
Since the general utility of bar codes for encoding information is widely recognized, it is not surprising that attempts have been made to mark bar codes on metal surfaces. Such attempts have met with only limited success in practice. Marking a bar code on a metal surface using laser etching or mechanical abrasion requires expensive equipment, and moreover poor read-rates sometime result. A bar code printed on a sticker and adhesively attached to a metal surface is easily damaged even if a tough material is used for the sticker. Furthermore the label may become detached due to degeneration of the adhesive if the surface is exposed to high temperatures or harsh chemicals. A bar code might be painted on a metal surface through a suitable stencil which is pressed against the surface, but paint seepage between the metal surface and the stencil at the edges of bars would result in poor read-rates. Furthermore the paint might chip, further reducing readability, if the surface is subjected to rough treatment. One might attempt to reduce the chipping problem, when a stencil bearing a bar code is pressed against a metal surface, by marking the surface using electrochemical etching rather than paint, but bars with poorly defined edges and hence poor readability would still result.
Electrochemical etching is a technique that is widely used to mark numbers or letters on a metal surface. Both ferrous metals, including stainless steel, and non-ferrous metals such as aluminum, brass, and copper, can be marked using electrochemical etching, as can plated layers of chrome, nickel, etc. Using the conventional electrochemical etching technique, one first prepares a stencil bearing the desired numbers or letters. Except at regions within the outlines of the desired numbers or letters, the stencil is impervious to fluid. After the metal surface to be marked is thoroughly cleaned, a pad which is soaked in an electrolyte is used to press the stencil against the surface. One output terminal of a power supply is then connected to the metal, and the other terminal is connected to a conductive plate within the pad. If the power supply delivers alternating current, during one portion of the waveform metal is etched away from the surface, where the surface is exposed to electrolyte through the stencil, and during the other portion of the AC waveform the metal is re-deposited as an oxide. The resulting metal oxide deposit is tough, and is easily distinguished visibly from adjacent portions of the metal surface which have been protected from the electrochemical etching by the impervious regions of the stencil. Direct current may be used instead of alternating current, in which case metal is etched away from the surface but not redeposited as an oxide. Morover the AC and DC techniques can be combined, so that the AC technique is used to deposit a metal oxide coating at the bottom of a recess which was produced using DC etching. The etching current is not critical, and typically ranges from one amper to thirty ampers. Electrochemical etching equipment, including stencil materials, power supplies, and suitable electrolytes for use with a wide range of metals, are commercially available. One such commercial source is Monode Marking Products, Inc., 23620 St. Clair Avenue, Cleveland, Ohio 44117.